Automatic headlight dimmer system



y 1, 1960 E. G. MATKINS 2,939,052

AUTOMATIC HEADLIGHT DIMMER SYSTEM Filed Dec. 5, 1956 /i 7 Z *L ayezze 7712559122.;

ATTORNEY United States Patent Oflice A 2,939,052 Patented "May 31, 1960 2,939,052 AUTOMATIC HEADLIGHT DINIMER SYSTEM Eugene G. Matkins, Reelsville, Ind., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Dec. 5, 1956, Ser. N0. 626,471 4 Claims. (Cl. 317-130 This invention relates to light-controlled switching means and more particularly to automatic light-controlled switching means for switching between high and low beam filaments on automotive vehicles. Present commercial automatic headlight dimming means for automotive vehicles embody electron tubes as amplifying means and therefore require the necessary high voltages for the electrode elements of the tubes. This necessitates power With the above and other objects in view which will.

become apparent as the specification proceeds, my invention will be best understood by reference to the following specification and claims and theillustrations in the accompanying drawings, in which:

Figure l is a circuit diagram of a light-controlled switching system embodying my invention, and

Figure 2 is a partial circuit diagram of the connections between the transistor amplifying stages and the relay showing a modified form of my invention.

Referring now more particularly to Figure 1, there is shown therein a pair of headlamps 2 and 4 such as those found on automotive vehicles in each of which there is provided a low-beam filament 6 and 8 and a high-beam filament 10 and 12 respectively. The two low-beam elements 6 and 8 may be sealed in separate lamps from the high-beam filaments if desired and are connected through common conductive line 14 to stationary contact 16 of a power relay 18 for switching between high and low filaments. In like manner upper beam filaments 10 and 12 are commonly connected through line 20 to stationary contact 22 spaced from stationary contact 16.

A movable switch armature 24 moves between stationary contacts 16 and 22 and engages one or the other to cause alternate illumination of the highor low-beam filaments. Armature 24. is springbiased upwardly as shown in Figure'l through spring 26 and is moved downwardly upon energization of the relay coil 28. One terminal of the coil 28 is grounded and theother terminal connected through conductive line 30 to movable arm 32 of the conventional foot switch 34 normally found on an automotive vehicle.

' The movable arm 32 engages either stationary contact 36 or spaced stationary contact 38 depending on the desire -of the operator and remains in contact with such stationary contact until physically moved to the other position 'as .desired- Armature24 'of the power relay 18 is directly connected to supply line 40 which is in turn connectedto the battery of the vehicle (not shown). Supply line 40 is also connected to connecting line 42 which extends from stationary contact 38 to supply line 40 and directly to the heating filaments 44 and 46 of the tube T-l. This tube T-1 is a double triode tube. Stationary contact 36 of the foot switch 34 is connected through line 48 to stationary contact 50 of a sensitive control relay 52 operable by the light-sensitive circuit. The sensitive relay 52 likewise incorporates a movable armature 54 which is spring biased upwardly by spring 56 and is moved downwardly away from engagement with contact 50 by'energization of relay coil 58. In its attracted position armature 54 engages stationary contact 60 of the sensitive relay 52, which contact 60 is connected through conductive line 62 and variable resistance 64 to the cathode 66 of the second section of the tube T1.. Armature 54 is directly connected through tieline 68 with power supplyline 42. A resistance 70 is connected between the lines 68 and 48. This much of the system involves the control of the power relay 18 by the sensitive relay 52.

When the sensitive relay 52 is energized to hold its armature in the lower position against the spring bias during no-light condition and with the manual foot switch 34 in the position shown, the circuit to the power relay coil 28 is broken; saidcoil being de-energized, spring 26 holds armature 24 in its upper position and therefore the upper beam filaments 10 and 12 are energized through an obvious circuit from supply line 40. If, however, sufiicient light falls on the control photocell, the current flow through coil 58 is reduced and when it releases armature 54, allowing the same to be pulled back to. its upper contact, an energizing circuit is completed to power relay coil 28 which attracts its armature 24 to de-energize the upper beam filaments 10 and 12 and energize lower beam filaments 6 and 8.

This light-sensitive control circuit will now be described. Relay coil 58 is in the output circuit of the transistor B 'and therefore when a suflicient amount of current flows through the series circuit including said transistor, coil 58 attracts its armature 54, but when this current is reduced, releases the armature 54 to its upper position. Coil 58 has one terminal connected through line 72 to the collector electrode 74 of the transistor B. The remaining terminal ofthe coil 58 is connected through variable resistor 76 to ground. The light-sensitive unit in this case is shown at 78 and includes what is currently known as a broad area cell 80. This lightsensitive unit is adapted to be mounted in a suitable housing having a condensing lens to concentrate light from a given direction on its surface. One terminal of the lightsensitive cell 80 is connected through line 82 directly to control grid 84 of the second triode section of the tube T-1 which is also connected through biasing resistor 86 to ground. The other terminal of the broad area cell 80 is connected through line 88 to plate 90 of the first triode section of the tube T-1. The cathode 92 of the first section is connected directly to ground together with the remaining terminal of the filament 46.

Since this circuit incorporates both transistors and tubes, it is necessary to provide some time delay means in initiating the operation to provide time for the tubes to warm up. The transistors, of course, may be instantly utilized upon the application of power. For time delay, a thermal switch S-l is provided, which includes a heating coil 94 connected to the power supply line 42 and ground and a bimetal switch arm 96 likewise connected to power supply line 42. This movable switch arm 96 is adapted to engage stationary contact 98 of the switch S-l when in heated condition and will move away from contact therewith when cooled. Stationary contact 98 is directly connected to secondary power supply line 100. Plate 90 of the first section of the tube T-1 is connected to secondary power supply line 100 through load resistance 102 and control grid 104 is connected to an intermediate point between resistors 105 and 106 connected in series between the secondary power supply line 100 and ground. This first section of the tube T-1 operates as a voltage regulating section.

The photocell unit 78 is connected between the first and second sections, said second section therefor acting as an amplifier, said photocell unit being connected directly between plate of the first or voltage-regulating section and the control grid 84 of the second or amplifying section. The plate 108 of the second section is directly connected through line 110 and limiting resistance 112 to the base electrode 114 of the first transistor amplifier A. The emitter electrodes 116 and 118 of the transistors A and B are commonly connected to the secondary power supply line 100. The collector electrode 120 of the transistor A is connected through biasing resistor 122 to ground and through limiting resistor 124 to the base electrode 126 of transistor B. A potential divider including series resistances 128 and 130 is connected between secondary power supply line 100 and ground. A variable tap 132 on resistor 130 is connected to the cathode 66 of the amplifier section of the tube T-l to provide a bias therefor.

In the operation of the system as shown in Figure 1, when the main lighting switch is closed to energize line 40, a circuit is immediately completed from the main power supply line 40 to the filaments 44 and 46 of the tube T-l to enable them to warm up. This circuit is as follows: supply line 40, line 42, filament 44, filament 46 and ground. At the same time a circuit has been completed to the heating element 94 of the time delay switch 8-1 which is obvious. Heat is therefore applied to the bimetal arm 96 of the switch S1 and at a predetermined time said arm will move into engagement with stationary contact 98. Such engagement will complete an energizing circuit to the secondary power line 100 applying power to the transistors and to the other elements of the tube T-.1 after the necessary delay.

The first section of the tube T-l has been described as being a voltage regulating section. Its operation may be described as follows: the cathode 92 is directly connected to ground and the plate 90 to the supply line 100 through a suitable load resistance 102. The control grid 104 is biased to a positive voltage by connection to the bleeder network 105-406. If the line voltage on line '100 is, for example, 13 volts, approximately 6 volts will appear at plate 90. When the line voltage changes to perhaps 15 volts, there is a tendency for the voltage on plate 90 to rise a proportionate amount. It is, however, prevented from doing this because of the simultaneous increased heater voltage on heaters 44 and 46 which increases tube conduction and thus provides more plate current flow to tend to reduce the plate voltage. The grid bias on grid 104 is selected to assist in this action and provide better regulation. On the other hand, as line voltage decreases, the reverse action occurs which tends to regulate the line voltage.

It is to be noted that the voltage of cathode 66 in the second triode stage of tube T-1 is determined by two resistors 130 and 64. Variable tap 132 is movable over the resistor 130 and resistor 64 is in itself variable. These two adjustments therefore determine the sensitivity of the amplifier. Resistor '64 is only in circuit with the cathode 66 when sensitive relay 52 is energized, whereasa portion of the resistance 130 is always in circuit with the cathode 66. Adjustable tap 132 may be termed the hold control, whereas variable resistor 64 is the dim control. These two settings determine the light level at which the control circuit switches to dim and the different light level at which it switches back to bright or high beam.

The operation of the circuit is as follows: assuming that the light switch has been actuated and that the tube has had an opportunity to warm up, and that switch S-1 is closed energizing all essential parts of the circuits, under no light or a small amount of ambient illumination, sensitive relay coil 58 has a sufiicient amount of current flowing therethrough to attract its armature 54 and hold it against the lower stationary contact 60. This is due to the fact that under no-light conditions the broad area cell has a very low level of current flow therethrough, therefore grid 84 connected to one end of the cell 80 is very nearly at ground potential. The cathode 66 at this time is biased above ground by an amount which is determined by the position of the tap 132 on resistance 130 and the setting of variable resistor '64. This provides a higher bias on the cathode and conduction through the second section of the tube T-l is essentially cut off. Since the base 114 of transistor A is connected directly in series with this tube section, there is practically no base current flow through transistor A. This in turn, of course, limits the collector current through transistor A. Base 126 of transistor B which is directly connected to collector of transistor A, is therefore connected to a low potential point since there is little current fiow through the collector circuit of transistor A. The potential on emitter 118 at this point is relatively high and this permits a sufficient base current flow in transistor B to provide collector current sufiicient to energize coil 58 to attract its armature 54. Thus, sensitive relay 52 is closed during no-light or limited-light conditions. When armature 54 is in the lower position, there is no energizing circuit for coil 28 of the power relay and armature 24 thereof is retained in its upper position by spring 26 and the upper beam filaments are energized.

As more light falls on the broad area cell '80, current flow through it increases, which increases the potential on control grid 84. When this reaches a critical point, conductivity through the second section tube T- l causes an increase in flow of base current through transistor A. In turn, this causes increased collector current in transistor A and this reduces the potential difference between base 126 and emitter 118 of transistor B. Collector-emitter current flow through transistor B is thus reduced causing coil 58 to drop armature 54 to break contact with front contact 60 and to engage rear contact 50. This movement has two results. First, it completes an energizing circuit for relay coil 28 as follows: Power line 40, line 42, line 68, armature 54, contact 50, line 48, contact 36, manual switch arm '32, line 30, coil 28 to ground. This energization causes armature 24 to be moved to the lower position, de-energizing the upper beam filaments and ener gizing the lower beam filaments. At the same time resistance 64 is removed from the cathode circuit of the second section of the tube T-1, leaving only that portion of the resistance 130 in that circuit to determine the potential of the cathode 66. This resistance 130 now determines amplifier sensitivity and that point to which the light intensity must fall before the device will switch back to high beam. It is a different and lower light intensity than that which caused the system to switch from high to low beam. This is necessary in order to prevent cycling or flashing when the oncoming vehicle in courtesy dims its lights and there is a reduction in light intensity falling on the control photocell immediately after the circuit has switched from high to low beam. Upon a reduction in light falling on the photocell, the reverse action takes place and relay '58 is again energized to cause a switching back to high beam. Actuation of manual switch '34 overrides the light-controlled system and provides low beam energization as long as arm 32 engages contact 38.

The circuit shown in Figure 2 is a slight modification of that shown in Figure l and incorporates the addition of a feedback or accelerating lock-in line. Only the portion ofv the circuit affected by the change is illustrated. Transistor A and transistor B are shown as in Figure 1 with through a biasing resistance 122 to ground and through a v limiting resistor 124 to base 126 of transistor B. Collector electrode 74 of B is connected through line 72 to one terminal of the operating coil 58 of the sensitivity relay 52 and the opposite terminal, as previously, through variable resistor 76 to ground. Line 110, extending from the plate 108 of the second section of tube T-l, is likewise shown connected through resistor I12 to base 11-4 of the transistor A. The additional feature shown in Figure 2 is a feedback line connected between the line 72 and line 110. It includes a limiting resistor 140 which has one terminal connected to line 72 and the opposite terminal connected to line 110. This feedback path accelerates or aids the increase or decrease of current to cause a more positive and quick actuation of the system. For example, if the collector current in transistor B begins to decrease due to an increasing amount of light falling on the broad area cell 80, this voltage decrease is coupled back through resistor 140 to the plate 108 of the tube T-1 through line 110. This feedback signal aids in the direction of voltage change andthe result is additive. This causes a quicker decrease in the current through coil 58 when light intensity increases and a more prompt and positive actuation of the circuit. The opposite action occurs when going from the light to the no-light condition as the feedback voltages aid in the change.

I claim:

1. A vehicle headlamp control system comprising a voltage source, a photocell adapted to develop a signal corresponding to the incident light intensity, a thermionic amplifier tube having an input circuit, output circuit, and heater circuit, said photocell being connected with the input circuit of the amplifier tube, a transistor amplifier having an input circuit and an output circuit, a relay device having an energizing coil, the input circuit of the transistor amplifier being connected with the output circuit of the amplifier tube, the output circuit of the transistor amplifier being connected with said energizing coil to control the relay device in accordance with the signal of the photocell, a time delay switch, said voltage source being connected directly with the heater circuit of the amplifier tube and being connected through said time delay switch to the output circuits of the transistor amplifier and the amplifier tube so that said relay device is de-energized until the amplifier tube is eflfective to permit control of the relay by the signal of the photocell.

2. The combination as defined in claim 1 wherein the time delay switch has a fixed contact, a bimetal movable contact, one of said contacts being connected to the voltage source and the other being connected to the output circuits of the amplifier tube and transistor, an electrical resistance heater adjacent the bimetal movable contact and connected with the voltage source.

3. The combination as defined in claim 1 wherein the photocell is a photoconductive device and wherein the control system includes an additional thermionic amplifying tube having a plate, cathode, grid, and heater, a volt age divider connected across said source, said grid being connected to an intermediate point on the voltage divider, said plate being connected to one terminal of the voltage source through a resistor and said cathode being connected to the other terminal of the voltage source, said heater being connected across the source whereby the thermionic emission of the cathode varies with voltage fluctuations of the source so that the voltage on the plate remains substantially constant, an input resistor in the input circuit of the first-mentioned amplifier tube, said photoconductive device being connected between the plate and said other terminal of the voltage source through said input resistor to provide a signal independent of fluctuations in the voltage of the source.

4. A vehicle headlamp control system comprising a voltage source, a photoconductive cell adapted to change conductivity in accordance with the incident light intensity, amplifying means having an input circuit including an input resistor and an output circuit, a relay device having an energizing coil connected with said output circuit to control the relay device in accordance with the signal of the photoconductive cell, a thermionic amplifying tube having a plate, cathode, grid, and heater, a voltage divider connected across said voltage source, said grid being connected to an intermediate point on the voltage divider, said plate being connected to one terminal of the source through a resistor and said cathode being connected to the other terminal of the source, said heater being connected across the source whereby the thermionic emission of the cathode varies with voltage fluctuations of the source so that the voltage on the plate remains substantially constant, said photoconductive cell being connected between the plate and said input resistor of the amplifying means to provide a signal independent of the voltage fluctuations of the source.

References Cited in the file of this patent UNITED STATES PATENTS 2,571,981 Wensel Oct. 16, 1951 2,623,170 Dickinson Dec. 23, 1952 2,628,310 Wood Feb. 10, 1953 2,685,665 Price Aug. 3, 1954 2,718,612 Willis Sept. 20, 1955 2,777,097 Atkins Jan. 8, 1957 2,786,964 De Witt Mar. 24, 1957 2,842,721 Atkins July 8, 1958 OTHER REFERENCES Transistorized Headlight Dimmer, Radio and Television News, pages 56-57, August 1955. 

